This invention relates generally to radio antennas and more particularly to antennas adapted to transmit and/or receive radio frequency energy with any one of a variety of polarizations.
As is known in the art, it is often desirable to use an antenna element which can operate with any one of a variety of polarizations, such as linear (i.e. vertical or horizontal) or circular. Often, an array of such antenna elements is used in order to provide a collimated and angularly directed beam of radiation and to attain a relatively wide scan angle (i.e. narrow beam). The angular direction of such beam is related to the phase angle distribution provided by a phase shifting network across the array. Thus, on either transmit or receive, radiation is fed to or received by the array with a phase distribution in accordance with a desired angular direction.
One such antenna arrangement is described in U.S. Pat. No. 3,836,976 entitled "Closely Spaced Orthogonal Dipole Array," with inventors George J. Monser, George S. Hardie, John R. Ehrhardt, and Jerry M. Smith, issued Sep. 17, 1974 and assigned to the assignee of the present invention. More particularly, an array of antenna modules is described, with each module including a pair of planar antenna elements. The pair of planar antenna elements intersects along a common line of intersection with the antenna elements being disposed orthogonally with respect to one another. Each planar antenna element of a module is fed by a separate radio frequency energy feed. For circular polarization, each of the feeds of the module is fed with radio frequency energy having a quadrature phase difference.
More particularly, each planar antenna element has a flared notch, symmetrically disposed with respect to the line of intersection. A forward portion of the flared notch is disposed along a forward edge of the planar antenna element, adjacent free space, so that radio frequency energy is coupled between the antenna element and free space therethrough. A rearward portion of the notch terminates in a relatively narrow slot which in turn is coupled to a coaxial transmission line. While the slot is disposed along the line of intersection, the coaxial line is displaced therefrom so that the pair of planar antenna elements can physically intersect. More particularly, the outer conductor of the coaxial line is connected to one side of the slot while the center conductor of the coaxial line crosses the line of intersection to span the slot for connection to the other side of the slot. Thus, on transmit, radio frequency energy fed to the coaxial transmission line produces an electric field across the slot; whereas on receive, radio frequency energy received by the notch produces an electric field across the slot for coupling to the center conductor of the coaxial transmission line.
However, because the center conductors of each of the pair of antenna elements of a module must be electrically isolated, they cannot occupy the same space and, thus, are displaced one from the other as they cross the common line of intersection to span their corresponding slots. Hence, the phase center (i.e. the point which, from the far field, appears as the source of RF energy) of each of the pair of planar antenna elements are at different locations, resulting in non-coincident phase centers. More specifically, the only way to realize coincident phase centers is through manipulation of the radio frequency energy fed to each of the orthogonal elements. Coincident phase centers are desirable so that an antenna module appears as a point source of radiation from the far field. While the above-described antenna may be useful in some applications, in other applications a higher degree of phase coincidence may be desirable without the use of energy feed manipulation.
In another type of antenna arrangement adapted to transmit and receive radio frequency energy with a variety of polarizations, each antenna module again includes a pair of orthogonal planar antenna elements, but with each such element having a pair of flared notches spaced by a predetermined distance along the forward edge of the antenna element. More particularly, a forward portion of each of the pair of flared notches of each antenna element is disposed along the forward edge of such element, adjacent free space. Each one of such notches again has a rearward portion terminating in a relatively narrow slot. Here however, each of the pair of flared notches of an element is positioned symmetrically, on opposite sides of the line of intersection.
More particularly, a single radio frequency feed is coupled to an input/output port of a power divider/combiner. Each one of a pair of output/input ports thereof is coupled to a different one of the pair of narrow slots and such slots ar disposed on opposite sides of the line of intersection. Thus, with a pair of intersecting planar antenna elements, there will be four spaced feed points for the four narrow slots. However, because of the symmetrical positioning of the pair of notches and narrow slots about the line of intersection, each pair of notches appears in the far field as emanating from a single point. Thus, from the far field, each element has the appearance of having a single phase center disposed on the line of intersection. Furthermore, with two intersecting planar antenna elements, there remains the appearance of a coincident phase center on the line of intersection. Moreover, such antenna elements are provided with coincident phase centers without manipulation of the radio frequency energy fed thereto.
However, it has been found that when arrays of such antenna modules are used, adjacent elements or modules may interact with one another because of cross-coupling effects. More particularly, energy transmitted by an antenna element may couple into an adjacent element and cause resonating in the adjacent element. Specifically, this problem occurs when the wavefront of the radiation is at an angular direction, other than normal, to the array. In such case, when the energy received by each of the pair of flared notches of the adjacent antenna element is out of phase, such energy does not fully combine in the power divider/combiner and such uncombined energy resonates in the element. Moreover, such resonating energy, often referred to as odd mode resonance, is undesirable since it may cause narrow band dropouts or scan blindness at the resonating frequency.
One technique known in the art for reducing the aforementioned resonating condition is to use a power divider/combiner which includes energy dissipating resistors to damp out the odd mode resonance (i.e. absorb the resonant energy). However, this technique may be costly due to the complexity associated with fabricating such a power divider/combiner, as well as the concomitant reduced production yield. Moreover, use of this technique limits the power handling capability of the power divider/combiner in accordance with the power handling capability of the damping resistors. Furthermore, the use of energy dissipating resistors reduces the overall efficiency of the antenna for certain operating conditions.